# 12.2  Factors that trigger epidemics

In Study Session 7 you learned about factors that affect the transmission of malaria. In this section you will learn how some of those factors are also associated with the occurrence of malaria epidemics.

The ‘host’ is the infected organism — in malaria, the host is always a human.

Malaria epidemics are triggered by factors linked to the human host, the mosquito vector (the environment) and malaria parasites, as you can see in Figure 12.1. The change or disruption of the ‘balance’, between these three factors at any one time may increase the likelihood of an epidemic. That is, there is an increased risk of a malaria epidemic, if there is an increase in:

• the susceptible human population
• the number of mosquito vectors
• an increase in the number of people who have the malaria parasite in their blood.

Figure 12.1  The three factors that affect the malaria transmission (host, parasite and environment).

You need to carefully and closely monitor changes in these factors in order to predict the risk of an epidemic. How some of these factors can be monitored is described next.

12.1  What is a malaria epidemic?

12.2.1  Environmental factors